I had exactly the same problem at a client today with a Windows 10, Dell Celeron Laptop.
The machine took almost 30 minutes to open Outlook and was terribly slow in general.
The CPU ran at 30% maximum utilization, at 0.48Ghz PERMANENTLY, every day.
I noticed at the right bottom of the screen that although the laptop was plugged into power, the battery icon said "not charging" and battery level was 1%.
To resolve it, I did the following (perhaps this could work for you too):
- Double-click the battery icon (not right-click) at the right-bottom of the screen (in the tray)
- Click "Battery Settings"
- Untick the option for: "Turn battery saver on automatically if my battery falls below..."
- Disable the setting: "Battery saver status until next recharge"
- Unplug the charging cable and re-plug it into the charging port of the laptop.
The battery icon at the right bottom of the screen should now change to "Charging".
After this the CPU started running at 100%, 2.48Ghz and the machine worked well again.
For good measure, click on the battery icon again and set the power mode to "Best Performance"
Note: I noticed that all the battery settings for "Pressing the Power Button", "Pressing the Sleep button" and "Closing the lid" were set to SLEEP. So I guess that every day when the client went home, he closed the lid and put it in his laptop bag. This totally drained the battery, and because the setting was enabled to use Battery Saving when power dips below 20%, every morning the laptop would be SUPER slow.
Whether the Battery Saver setting disabled the battery from charging I can't say for sure.
To avoid the problem for this client, I changed most settings to "Shutdown" and some to "Hibernate"